The goal of this project is to study retail drug market characteristics and their relationship to drug use patterns, medical consequences of drug use and cross-border mobility among injection drug users (IDUs) in the southwestern U.S.-Mexico border region. Mexico is the largest foreign supplier of illicit drugs to the U.S. Tijuana, the sister city of San Diego, CA, is home to the busiest border crossing in the world and is situated on a major drug trafficking route whereby heroin, methamphetamine (meth) and cocaine have created thriving local consumption markets. Little is known about local trends in retail drug markets, their antecedents and impacts in this region, which has a bearing on both countries. We will leverage our existing binational research infrastructure to conduct surveillance of specific drug market characteristics in a timely, cost-efficient manner to address these aims: 1) To describe perceived and actual trends in drug market characteristics (e.g., purity, price, availability and formulations of heroin, meth and cocaine) and their relationship to drug use patterns in San Diego and Tijuana;2) To identify direct and indirect associations between specific drug market characteristics and negative medical consequences (e.g., soft tissue infections, overdose, mortality) among IDUs in both cities;3) To study trends in bi-directional cross-border mobility of IDUs in San Diego and Tijuana in relation to drug market characteristics, drug use patterns, involvement in the drug economy and intensity of border security;4) To compare specific drug market characteristics, drug use patterns, involvement in the drug economy, medical consequences of drug use and cross-border mobility among IDUs in San Diego and Tijuana, overall and in relation to social environmental influences (e.g., changes in policing and border security). To meet the proposed aims, we will recruit 600 IDUs (300 IDUs each from San Diego and Tijuana) aged 18 or older who have injected heroin, meth or cocaine in the last month using respondent driven sampling. IDUs will undergo interviews at baseline and at 6, 12, and 18 months. We will also obtain data on purity, price and availability of heroin, meth and cocaine from various external sources to triangulate perceived drug market trends from self-reports. To address Aims 3 and 4, we will also obtain records of U.S. border patrol line-watch hours for specific border sectors and systematically record counts of police activity/arrests at specific intersections over time in both cities. In meeting these aims, this study will identify important trends on retail drug markets, their antecedents and potential sequelae in the Mexico-U.S. border region during a period of intensified policing and border security. Given that northwestern Mexico represents a critical corridor through which illicit drugs enter the U.S., this study represents an unprecedented opportunity through which we can inform drug abuse prevention, treatment and drug policy in both the U.S. and Mexico.